The overall purpose of this competitive revision application is to expand our knowledge of the socio-sexual networks of young men who have sex with men (YMSM). Basic research on these networks, and the degree of their overlap, will increase our understanding of network processes related to a cluster of health issues disproportionally experienced by YMSM. This group has an alarming HIV/AIDS prevalence, and is one of the only risk groups showing an increasing rate of infections. This epidemic of HIV among YMSM does not exist in a vacuum;YMSM experience multiple health disparities, including elevated rates of illicit drug use, exposure to violence, and mental health problems, know as a syndemic, which is associated with HIV risk. Under the parent award on which the revision application is based, we have begun to gather of sample of 450 YMSM, ages 16-20, using respondent-driven sampling (RDS) to investigate this syndemic of psychosocial health issues linked to HIV and characterizing multiple trajectories of syndemic development across 2 years. The two overarching goals of the parent study are to: 1) to provide much-needed epidemiological data on the prevalence of HIV and related health issues in order to inform public health priorities;and 2) to collect vital information on individual-level risk and protective factors to inform the development of an intervention targeting this vulnerable population. In keeping with the goals of NOT-OD-10-033, "NIH Announces the Availability of Recovery Act Funds for Competitive Revision Applications (R01, R03, R15, R21, R21/R33 and R37) for AIDS- related Research through the NIH Basic Behavioral and Social Science Opportunity Network (OppNet)," we intend to conduct research that advances our basic understanding of social systems of relevance to HIV risk and prevention. Network-based research is a priority area of HIV-related investigation, however very little is known about the basic structure and characteristics of these networks or their influence on HIV-related risk and other clustering health issues in YMSM. Social support theory and the growing literature on sexual network epidemiology guide our two aims: 1) to identify and describe the socio-sexual network structure of YMSM;2) to establish the comparative value of social and sexual network methods, versus traditional individual-level measures, in predicting individual syndemic outcomes. Our network-based study reflects basic behavioral and social sciences approaches to understanding socio-sexual network processes, and will inform the field about the comparative value of this more resource-intensive methodology. Novel features of this study include assessment of three types of network structure (social, sexual, substance using) integrated into the parent study, which includes longitudinal assessment of psychological and HIV/STD biomarker outcomes and processes. Findings will be used to guide future prevention and intervention efforts by identifying network factors and processes that influence risk from mid/late adolescence through emerging adulthood to reduce extremely high rates of HIV infection in this population. PUBLIC HEALTH RELEVANCE: Young men who have sex with men (YMSM) have an alarming HIV/AIDS prevalence, and are one of the only risk groups showing an increasing rate of infections in recent years. The overall purpose of this competitive revision application is to expand our knowledge of the social and sexual networks of young men who have sex with men (YMSM) to advance our basic understanding of social systems of relevance to HIV risk and prevention.